Regarding the invalid grant_type, please see:
https://github.com/openhab/openhab-addons/pull/13050
As you probably can’t wait for 3.4 in December, you can grab the last JAR here:
https://ci.openhab.org/job/openHAB-Addons/lastSuccessfulBuild/artifact/bundles/org.openhab.binding.gardena/target/org.openhab.binding.gardena-3.4.0-SNAPSHOT.jar
Regarding the 429 (“Limit Exceeded”) error, please follow this thread:
https://community.openhab.org/t/gardena-error-429-limit-exceeded/